When a density of a shoe sole made of a polyurethane foam is lowered in order to reduce its costs and weight, its mechanical strength is lowered. Therefore, in order to improve the mechanical strength of the polyurethane foam, there has been employed the addition of a cross-lining agent, a polyfunctional polyol, an aromatic polyol or the like. Also, since the strength at the initial of reaction is lowered along with lowering of the density, the time period required for demolding a molded article such as a sole from a mold (hereinafter referred to as “demolding time”) becomes long. As its countermeasure, the amount of a catalyst has been generally increased.
Another problem caused by the reduction of weight is that voids (air-accumulation generated in the internal of the molded article) or air-lack (surface defects caused by insufficient filling of a liquid molding material and/or a foamed article in a complicated internal shape of a mold) is generated, so that insufficient filling is apt to cause molding failure. In many cases, these are caused by the insufficient filling of a liquid molding material into a mold due to its small amount injected. This problem becomes notable especially when a midsole having a complicated shape is produced or when a sole having a density of the molded article of not more than 0.30 g/cm3 is produced.
In order to avoid the molding failure, there has been generally employed a process for suppressing the reactivity at the initial stage by reducing the amount of a catalyst. However, there are some defects in this process such that the productivity is lowered, since the exhibition of the initial strength is delayed, to prolong the demolding time.
Therefore, conventionally, it has been difficult to satisfy both productivity and moldability.
Incidentally, there has been known a process for producing a non-flexible polyurethane foam by using a low-molecular weight polyol together with a high-molecular weight polyol, and simultaneously using not less than 1 part by weight of urea and not less than 2 parts by weight of water per 100 parts by weight of the polyol (Japanese Patent Laid-Open No. Sho 61-268716). However, an object of this process is to increase gas permeability. Therefore, this process does not suggest a process for improving strength and moldability of the polyurethane foam using urea. Moreover, since the polyurethane foam obtained by this process is not flexible but brittle, the polyurethane foam is completely unsuitable for shoe soles.